1. Technical Field of the Invention
The present invention relates to a laser-diode-pumped solid-state laser apparatus and a status diagnostic method of the apparatus. Particularly, the present invention relates to a highly reliable laser-diode-pumped solid-state laser apparatus and a status diagnostic method of the apparatus, which are capable of performing degradation diagnosis for a laser diode simply and easily.
2. Description of the Related Art
A so-called LD (laser diode) pumped solid-state laser, which has a laser diode (hereinafter, abbreviated as LD) with higher absorption efficiency to a laser medium than a lamp and is small, highly efficient and has a long lifetime as a pumping light source, has drawn attention in recent years as a method of pumping light from a solid-state laser medium such as an Nd:YAG. Particularly, an LD-pumped solid-state laser apparatus has been developed in recent years, which emits a laser output reaching a kilowatt by using a few hundred LD""s in one resonator.
It is believed that the LD has a more than ten times longer lifetime than the lamp and the LD can be continuously used for as much as 10,000 hours. However, the lifetime is an average and output from some LD""s reduces after a few thousand hours, and it is difficult to completely recognize and remove them at initial LD selection. Further, since the LD reduces its lifetime considerably due to disturbances and changes in the external environment, such as static electricity, electric surges from a power source, return light, dust, gas and condensation, it is necessary to detect a quantity of a light pumped from the LD and know a degree of its degradation by some means in order to improve the reliability of the laser apparatus and to deal with a failure quickly.
Accordingly, in the LD for low output operation at 10 W or less, a photo-detector disposed in an LD package detects the energy of the pumping light that slightly leaks from a high reflection surface (rear surface) opposite to an emission surface (front surface) of an LD chip, and it is used for controlling a pumping light quantity or detecting degradation of the LD.
Further, a method has conventionally been used in which a laser oscillation light emitted outside the resonator from an output mirror that composes a solid-state laser resonator is partially split or the photo-detector measures the energy of the oscillation light leaked from a mirror other than the output mirror, thus controlling the laser output or detecting the degradation of LD.
Furthermore, a method has also been proposed, which detects fluorescence intensity or fluorescence distribution in a direction along a laser oscillation optical axis or on its extension. FIG. 1 and FIG. 2 show the method described in Japanese Patent Laid-Open (unexamined) No. 2000-269576. A method is proposed in which a monitoring mirror splits fluorescence emitted from a solid-state laser rod along the laser oscillation optical axis, a CCD camera transforms its pumping distribution into an image for observation, and a drive current for each LD is adjusted individually based on the image to unify the pumping distribution. The prior art will be described as follows.
In the conventional example, FIG. 2 shows a following method. When setting a value of the drive current for each of LD""s 102 to 107, excitation is performed first in the state where the mirror of the solid-state laser resonator is removed, variable resistors 122 to 127 adjust the values so as to unify the pumping distribution while a CCD camera 130 observes the pumping distribution, and the resonator mirror is attached again. Further, as shown in FIG. 1, a method is also proposed that a mirror 128 is inserted in the resonator only when measuring the fluorescence distribution in the state where the resonator mirrors 120, 121 are attached, and the CCD camera 130 observes the fluorescence from a laser rod 101 through the mirror 128.
Furthermore, this conventional example describes another method of making a drive power source 117 for LD small, simplifying wirings, and adjusting the drive current for each of the LD""s 102 to 107, in which all LD""s are connected in series and driven by one power source 117, and variable resistors 122 to 127 are connected to each of the LD""s 102 to 107 in parallel, and thus controlling a current value that flows in each LD.
However, an LD chip for high output operation at 20 W or more, which has been widely marketed, has the emission surface of about 1 cm in length. Accordingly, a wide detector is required to detect all the light from the width of 1 cm by the photo-detector provided in a rear position, and output from the photo-detector saturates unless a neutral-density filter is provided before the photo-detector because much light quantity leaks from the rear surface of the chip. As a result, since the configuration for light detection becomes complicate and its cost also increases, the LD for high output operation is not regularly provided with a mechanism for detecting the light quantity in its package in many cases. Moreover, in the case of a configuration called a stack where the LD""s are laminated for a semiconductor layer with a narrow gap in a perpendicular direction, photo-detection function is not regularly provided due to little space for providing the photo-detector on the rear surface. For this reason, it has been impossible to directly detect the degradation of LD in the case of a high output solid-state laser apparatus using the high output LD""s.
Optical output that can stably operate the LD device having the emission surface of 1 cm in length is from 20 W to the maximum level of about 80 W. Accordingly, a plurality of the LD""s need to be used in the case of the high output laser apparatus that requires higher excitation. For example, there is a case where a few tens to a few hundred or even more LD""s are used as a pump source with respect to a solid-state laser medium to obtain the laser output in the kilowatt class from the solid-state laser apparatus. When a very large number of LD""s are simultaneously used, the number of power sources becomes large if each LD device is individually provided with the drive power source, a wide installation volume is needed, the wirings become complicate, and thus leading to low operation efficiency. Therefore, a few to a few tens of LD""s are driven by connecting them electrically in series as a group regularly.
As described, when a large number of LD""s are driven in series, it is impossible to increase the drive current for a particular LD to compensate the output reduction of the LD even if individual LD is provided with the photo-detection function and the function can detect the output reduction of the particular LD among the LD""s, because a plurality of LD""s are electrically connected in series. For example, when compensating the output of the particular LD by increasing the drive current, the output of the other LD""s having no output reduction becomes larger than an initial value. Thus, the pumping light output of the whole group increases remarkably, which may lead to increase of the solid-state laser output exceeding an initially set value. In other cases, excitation balance with the LD""s of another group becomes unstable, which may conversely lead to reduction or instability of the solid-state laser output or quality reduction of an emission beam. Moreover, when each of a few tens of LD""s is provided with the photo-detector, wirings and circuits for controlling them become very complicate, and thus increasing the apparatus cost.
In addition, as a well-known conventional method, it is possible to detect laser oscillation light quantity taken outside the solid-state laser resonator to know indirectly the degree of LD degradation. However, the oscillation light quantity form the resonator does not reduce only due to the LD degradation but also reduces considerably due to alignment slippage of the resonator mirror, stain and damage of the mirror itself, alternation and damage of the laser medium, stain of its coating film or the like. It is difficult to know separately the degree of LD degradation from the light quantity of the laser oscillation light, because the oscillation light quantity is reduced more frequently by a cause other than the LD.
Further, in the conventional methods shown in FIGS. 1 and 2, since the CCD camera needs to be installed along the laser optical axis or on its extension and the fluorescence distribution cannot be observed due to an intense laser beam when the laser is oscillated, the resonator mirror needs to be removed once during measurement of the fluorescence distribution or the mirror is temporarily inserted in the resonator to keep the solid-state laser apparatus from oscillating, which is not practical because the LD degradation cannot be detected during operation. In addition, when a plurality of laser rods are arranged in the laser apparatus in order to obtain high laser output, it is extremely difficult for one CCD camera to observe the fluorescence distribution in a long laser rod or a plurality of laser rods at once from one direction along the laser oscillation optical axis due to a focal distance of imaging and the like. For this reason, a plurality of CCD""s, a plurality of mirrors, imaging lenses with various focal distances and the like are required, and thus an optical system becomes very complicate and its cost becomes high.
Moreover, when all the LD""s are connected in series and driven by one power source as shown in the conventional drawings, the number of LD""s connected in series becomes 20 to more than 30, and the power source becomes very large conversely if a drive voltage exceeds 40V. This is because electronic parts composing the power source become large quickly to cope particularly with an increase of the drive voltage. When the number of LD""s to be mounted is large, the power source is individually provided for 20 to 30 LD""s as one group to make the overall size of the power source small. Furthermore, one power source deals with a large current and a high voltage, a cooling method of the electronic parts, reduction of durability and reliability, and difficulty of replacement and maintenance due to large size of each part are large problems practically. In addition, when the variable resistors control the drive current for each LD, problems such as heating and drift in the resistor and reduction of operation efficiency also occur.
The findings that the inventor has obtained after building a prototype of the laser apparatus is that changing the current value of each LD causes only a small amount of change affecting the entire fluorescence distribution when one or a few laser rods are excited using a few tens to a few hundred LD""s. On the other hand, the mechanism for controlling the drive current for each LD does not function effectively since time fluctuation and thermal fluctuation of the current value supplied from the power source to each LD, ripple, and a fluctuation amount of fluorescence affected by detection fluctuation of a detector or noise are larger.
Moreover, a problem caused when all LD""s are driven by one power source is that all LD""s are broken simultaneously in a same manner if an excessive current exceeding an allowable range of the LD flows by a defective part in the power source, malfunction of a control system, an operation error by an operator, and an external factor such as thunderbolt and power out.
The primary object of the present invention is to provide a laser apparatus capable of grasping the degree of LD degradation constantly and accurately in an LD-pumped solid-state laser apparatus. Particularly, the object of the present invention is to provide a highly reliable laser apparatus capable of grasping and detecting the degree of LD degradation and the position of degradation accurately and with a simple configuration in the solid-state laser apparatus that mounts a large number of LD""s from a few tens to a few hundred pieces.
Further, another object of the present invention is to provide a simple method of correcting and adjusting the degree of excitation to the solid-state laser medium in accordance with the degree of a degraded LD.
According to the present invention, there is provided a solid-state laser apparatus that has a solid-state laser medium, which absorbs a pumping light to generate or amplify beam of a predetermined wavelength, and a laser diode light source, which generates the pumping light and introduces the generated pumping light into the laser medium directly or via an optical device. The apparatus includes fluorescence detection unit for detecting a quantity of fluorescence generated from the solid-state laser medium at a position inside a laser resonator composing the solid-state laser apparatus, the position being near an optical axis of a laser oscillation light generated in the resonator and not blocking the optical axis. The fluorescence detection unit can be composed of optical unit disposed in a position near the optical axis of the laser oscillation light and not blocking the optical axis and a photo-detector that detects the fluorescence introduced by the optical unit.
Detection of LD degradation in the present invention is performed by measuring not the laser oscillation light but the light quantity of the fluorescence emitted from the laser medium. Since the fluorescence quantity is proportional to the pumping light quantity absorbed in the laser medium, it includes effect and information of changes of an absorption quantity due to changes of wavelength. Particularly in the case of the LD, unlike the case of the lamp, it is possible to know the status of excitation and wavelength change caused by the LD degradation more accurately than simply measuring the pumping light quantity emitted from the LD since the wavelength changes greatly by an operation temperature.
Additionally, the light quantity of fluorescence is not affected by the alignment slippage, stain and the like of the resonator mirror at all. Although the light quantity reduces due to the alternation and damage of the laser medium itself, the stain of its coating film or the like, the degree of reduction is far smaller than the changes of the laser oscillation light. This is because the fluorescence is emitted from the entire laser medium on which the pumping light is irradiated and is harder to be affected by local changes than the laser oscillation light.
However, unlike the laser beam, since the fluorescence has weak directivity and spreads quickly for dissipation as it propagates after generation, it is desirable to detect the fluorescence at a position as close to the laser medium as possible. Although the outside periphery of the solid-state laser medium is normally covered with metal or the like so as to block the fluorescence, the laser medium is optically exposed near the optical axis of the laser oscillation light, where the fluorescence can be observed without fail. However, it is desirable to measure the fluorescence in the laser resonator because the fluorescence is also reflected by the resonator mirror outside the resonator even in the vicinity of the optical axis.
Herein, in the case of measuring the fluorescence quantity in the resonator, laser oscillation itself is inhibited if a photo-detector for measurement, the optical unit for guiding the fluorescence into the unit, or the photo-detector itself blocks the optical axis of the laser oscillation light. Accordingly, they need to be positioned as close to the optical axis as possible so as not to block it in order to constantly detect the fluorescence quantity in the operation state.
The findings that the inventor has obtained from experiments regarding the pumping distribution of the pumping light in the laser medium after building a prototype of the laser apparatus is as follows. Once a mechanical positional configuration between the LD and the laser medium is determined, a fluorescence quantity or a relative value thereof may only be detected, and there is no need to detect the pumping distribution itself by the CCD camera. Furthermore, excitation efficiency is made clear when the fluorescence quantity is measured, and it has made clear that the status of pumping distribution is kept under a constant state if the efficiency value is constant. Therefore, the fluorescence quantity needs not to be detected on the optical axis of the laser beam, but may be detected at a remote position not blocking the optical axis of the laser beam taking advantage of its property of spreading wider than the laser beam. Further, since measuring constantly the fluorescence at the same position maintains the ratio to the entire fluorescence quantity at a constant level, it is possible to accurately know the status of the entire excitation.
The present invention can have a configuration that the pumping light is introduced from a direction approximately perpendicular to the optical axis of the laser oscillation light.
When the pumping light from a larger number of LD""s is irradiated on the solid-state laser medium, a method called a so-called side pumping is effective, in which the optical axis of the pumping light is arranged perpendicular to the optical axis of the laser oscillation light in the solid-state laser medium and along the optical axis of the oscillation light. In the present invention, measurement can be performed without depending on the positional relation between the LD and the solid-state laser medium or a pumping method, because the pumping light from the LD is not directly measured but the fluorescence quantity from the solid-state laser medium is measured. The measurement method can be applied to the case where the optical axis of the pumping light is set perpendicular to the optical axis of the laser oscillation light in the solid-state laser medium.
Further, the present invention can have a configuration that the optical unit includes a mirror for reflecting the fluorescence emitted from the solid-state laser medium and the fluorescence reflected by the mirror propagates in space and is made incident into the photo-detector disposed in a predetermined position.
When the photo-detector is directly adjacent to a laser oscillation optical path in the laser resonator, there are cases where it cannot be made closer due to the shape and the size of the photo-detector or an installation position is limited. Then, the photo-detector is installed in a remote position from the optical axis of the laser oscillation light, the fluorescence from the laser medium is irradiated on the optical axis of the laser oscillation light, and unit for reflecting the irradiated fluorescence toward the photo-detector is provided. Thus, the fluorescence can be detected without directly positioning the photo-detector adjacent to the laser oscillation optical path.
Furthermore, the present invention can have a configuration that the optical unit includes a transparent medium in the wavelength of the fluorescence, and the fluorescence made incident from one end of the medium propagates through the medium and is made incident into the photo-detector disposed at the other end.
Because of the weak directivity of the fluorescence, the fluorescence spreads quickly when it is propagated in space to the photo-detector installed at a remote position from the laser oscillation optical path, and there are cases where sufficient light quantity for detection cannot be obtained. Accordingly, the present invention is the one, in which the transparent medium is used in the wavelength of the fluorescence, a portion of the medium is made adjacent to the laser oscillation optical path and the photo-detector is disposed such that another portion of the medium receives the fluorescence propagated through the medium. Thus, the fluorescence is confined within the medium when propagating, and does not spread nor dissipate considerably until it reaches the photo-detector. At this point, a portion of the medium that is made closer to the laser oscillation optical path is processed thin or finely so that it can be easily inserted in or adjacent to the laser optical axis in a narrow laser resonator. Furthermore, the surface shape of the medium, which is made closer to the laser oscillation optical path, may be processed or coating having a predetermined reflectivity may be applied to the surface such that the fluorescence is efficiently introduced in the medium and efficiently propagated to the photo-detector.
Furthermore, the present invention can also have a configuration that the optical unit is formed so as to surround the periphery of the optical axis in a part of the optical path of the laser oscillation light, and the fluorescence made incident from an end portion of an opening provided in the optical unit is guided into the photo-detector disposed in a predetermined position on an end portion of an outer periphery. The transparent medium is disposed in the wavelength of the fluorescence so as to surround the periphery of the optical axis, and thus even weak fluorescence can be effectively collected and taken out for detection.
Still further, the present invention can also have a configuration that the diameter of the opening is set smaller than that of the laser medium. Control of the oscillation transverse mode of the laser oscillation light and separation of the return light from outside the resonator can be performed simultaneously with detection of the fluorescence by allowing the medium to surround the periphery of the oscillation optical axis in the laser resonator as close as possible. For example, in Japanese Patent Laid-Open (unexamined) No. 10-253116, a circular aperture of a non-absorption type, which is disposed on a laser optical path to separate a main beam from a stray light component, is described. The fluorescence propagating through the aperture also can be detected by disposing the photo-detector adjacent to the external side of the aperture.
Further, in the present invention, it is desirable that the transparent medium is made up of glass as parent material. When glass is the parent material, it is inexpensive and processing of a shape is easy, and additionally, the fluorescence can be propagated through the medium to the photo-detector without being absorbed.
Furthermore, the present invention can also have a configuration that a filter selectively attenuates light having the wavelength of the pumping light is included on the optical path of the fluorescence reaching the photo-detector.
The pumping light might be simultaneously made incident on a photo-detecting surface for detecting the fluorescence emitted from the solid-state laser medium, depending on an excitation mode. When the laser oscillation optical axis and the optical axis of the pumping light are parallel with each other, a large quantity of the pumping light that has not been absorbed in the laser medium might go outside the laser medium to be made incident on the photo-detecting surface of the photo-detector simultaneously with the fluorescence from the laser medium.
In this case, in the case where the wavelength of the pumping light is changed to go off from the absorption wave length of the laser medium, the light quantity of the pumping light leaking outside increases because the quantity absorbed by the solid-state laser medium reduces despite that the fluorescence quantity from the laser medium reduces. Thus, the overall light quantity reaching the photo-detecting surface of the photo-detector changes greatly due to a total of the both light quantities, and it is detected on an increased quantity in some cases. Accordingly, correct information that the fluorescence has reduced cannot be sent. Therefore, a filter that selectively attenuates the light having the wavelength of the pumping light is included on the optical path where the fluorescence reaches the photo-detecting surface of the photo-detector, by which only the fluorescence can be received selectively.
Furthermore, the present invention can also have a configuration that a filter, which selectively transmits a wavelength of a fluorescence emission line spectrum that is not used in the laser oscillation out of the fluorescence emitted from the solid-state laser medium, is included on the optical path of the fluorescence reaching the photo-detector.
When oscillation of the solid-state laser apparatus begins, not only the fluorescence but also very intense diffused light from the oscillation light is irradiated on the photo-detecting surface of the photo-detector that detects reflected light or waveguide light from a position near the optical axis or the optical axis. Then, when output of the photo-detector saturates or gain of a signal amplifier is reduced to avoid saturation, there are cases where the fluorescence intensity before oscillation cannot be measured accurately. Further, when the oscillation light made incident into the photo-detector is very intense, it might break the photo-detector itself or deteriorates its performance.
Therefore, a filter that transmits only the wavelength of the fluorescence spectrum not used for laser oscillation is included, taking advantage of original divergence of the fluorescence emitted from the solid-state laser medium. Accordingly, the oscillation light does not saturate nor break the photo-detector since the fluorescence quantity made incident into the photo-detector does not increase greatly even if the laser medium oscillates.
Furthermore, the present invention can also have a configuration that the laser diode light source includes: a plurality of laser diode devices; a power source that drives a plurality of the laser diode devices in a predetermined number of groups; and controller for controlling the drive current of the power source, in which the controller adjusts the drive current for each group in accordance with the intensity of the fluorescence detected by the photo-detector.
As described in the foregoing conventional example, when a great number of LD""s are simultaneously used for excitation in the solid-state laser apparatus, a few to a few tens of LD""s are electrically connected in series and driven as one group generally. However, the LD""s are not only driven in groups but an electric circuit is also constituted such that each LD can be individually operated on the power source for each group, and the fluorescence quantity from the solid-state laser medium when driving each LD is individually is measured. Thus, operation status such as output and wavelength of each LD or the LD""s in a group can be individually detected and controlled, and a failure LD or a group including the failure LD can be identified.
Still further, only the failure LD and the LD""s in the minimum number of groups that include the failure LD are replaced, and thus the laser apparatus can be recovered without replacing other normal LD""s and LD""s in a group whose LD""s are all normal. As a result, replacement and maintenance operation of LD becomes remarkably easy, and parts cost of the LD and the like required in replacement also becomes inexpensive because only the minimum number of LD""s are required for replacement.
Moreover, not only when the LD has failure but also when the LD is slightly degraded, each LD or the electric circuit of a group including the LD is driven individually, and a control circuit is provided, which changes the drive current for each LD light source or its group in accordance with the intensity of the fluorescence generated from the solid-state laser medium, which is detected when driving the LD or the circuit. Accordingly, the excitation status for the solid-state laser medium can be recovered to a uniform status or a stable status initially set. Regarding the difference in absorption efficiency of the pumping light, which occurs due to dispersion of individual output and oscillation wavelength that the LD originally has, controlling the drive current based on the fluorescence intensity at the time of excitation of the LD can correct the dispersion within the minimum range for practical use.
Furthermore, there is no need to individually control the drive current for each LD, but one power source drives single LD""s up to about twenty pieces in a group and the circuit is not constituted such that each LD in the group can be individually driven. With this configuration, a small and highly reliable power source can be constituted, and failure of LD can be controlled in the minimum level even if an electronic part of the power source has a problem.
Still further, the present invention provides not only the laser-diode-pumped solid-state laser apparatus of the foregoing configuration, but also a diagnostic method of the degradation status of the laser diode light source performed by the laser apparatus.